1. Field of the Disclosure
The present disclosure relates to radio broadcast receivers and, in particular, to methods and systems for fine alignment of analog and digital signal pathways in a radio receiver.
2. Background Information
Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital radio broadcasting, uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
IBOC digital radio broadcasting signals can be transmitted in a hybrid format including an analog modulated carrier in combination with a plurality of digitally modulated carriers or in an all-digital format wherein the analog modulated carrier is not used. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog-to-digital radio while maintaining their current frequency allocations.
One feature of digital transmission systems is the inherent ability to simultaneously transmit both digitized audio and data. Thus the technology also allows for wireless data services from AM and FM radio stations. The broadcast signals can include metadata, such as the artist, song title, or station call letters. Special messages about events, traffic, and weather can also be included. For example, traffic information, weather forecasts, news, and sports scores can all be scrolled across a radio receiver's display while the user listens to a radio station.
IBOC DAB technology can provide digital quality audio, superior to existing analog broadcasting formats. Because each IBOC DAB signal is transmitted within the spectral mask of an existing AM or FM channel allocation, it requires no new spectral allocations. IBOC digital radio broadcasting promotes economy of spectrum while enabling broadcasters to supply digital quality audio to the present base of listeners.
Multicasting, the ability to deliver several audio programs or streams over one channel in the AM or FM spectrum, enables stations to broadcast multiple streams on separate supplemental or sub-channels of the main frequency. For example, multiple streams of data can include alternative music formats, local traffic, weather, news, and sports. The supplemental channels can be accessed in the same manner as the traditional station frequency using tuning or seeking functions. For example, if the analog modulated signal is centered at 94.1 MHz, the same broadcast in IBOC digital radio broadcasting can include supplemental channels 94.1-1, 94.1-2, and 94.1-3. Highly specialized programming on supplemental channels can be delivered to tightly targeted audiences, creating more opportunities for advertisers to integrate their brand with program content. As used herein, multicasting includes the transmission of one or more programs in a single digital radio broadcasting channel or on a single digital radio broadcasting signal. Multicast content over IBOC digital radio broadcasting transmissions can include a main program service (MPS), supplemental program services (SPS), program service data (PSD), and/or other broadcast data.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an BOO standard, designated NRSC-5A, in September 2005. NRSC-5A, the disclosure of which is incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. Copies of the standard can be obtained from the Consumer Electronics Association, Technology & Standards Department, 1919 S. Eads St., Arlington, Va. 22202 and the National Association Of Broadcasters, Science and Technology Department, 1771 N Street, NW, Washington, D.C. 20036. iBiquity's HD Radio technology is an implementation of the NRSC-5A IBOC standard. Further information regarding HD Radio technology can be obtained from iBiquity Digital Corporation, 6711 Columbia Gateway Drive, Suite 500, Columbia, Md. 21046.
Other types of digital radio broadcasting systems include satellite systems such as Satellite Digital Audio Radio Service (SDARS, e.g., XM Radio™, Sirius®), Digital Audio Radio Service (DARS, e.g., WorldSpace®), and terrestrial systems such as Digital Radio Mondiale (DRM), Eureka 147 (branded as DAB Digital Audio Broadcasting®), DAB Version 2, and FMeXtra®. As used herein, the phrase “digital radio broadcasting” encompasses digital audio and data broadcasting including in-band on-channel broadcasting, as well as other digital terrestrial broadcasting and satellite broadcasting.
Radio signals are subject to intermittent fades or blockages that must be addressed in broadcasting systems. Conventionally, FM radios mitigate the effects of fades or partial blockages by transitioning from full stereophonic audio to monophonic audio. Some degree of mitigation is achieved because the stereo information which is modulated on a sub-carrier, requires a higher signal-to-noise ratio to demodulate to a given quality level than does the monophonic information which is at the base band. However, there are some blockages which sufficiently “take out” the base band and thereby produce a gap in the reception of the audio signal. IBOC DAB systems can mitigate even those latter type outages in conventional analog broadcast, at least where such outages are of an intermittent variety and do not last for more than a few seconds. To accomplish that mitigation, digital audio broadcasting systems may employ the transmission of a primary broadcast signal along with a redundant signal (e.g. a digital OFDM signal and an analog AM or FM signal), the redundant signal being delayed by a predetermined amount of time, on the order of several seconds, with respect to the primary broadcast signal. A corresponding delay is incorporated in the receiver for delaying the received primary broadcast signal. A receiver can detect degradation in the primary broadcast channel that represents a fade or blockage in the RF signal, before such is perceived by the listener. In response to such detection, the delayed redundant signal can be temporarily substituted for the corrupted primary audio signal, acting as a “gap filler” when the primary signal is corrupted or unavailable. This provides a transition function for smoothly transitioning from the primary audio signal to the delayed redundant signal.
One example of transitioning from a digital signal to an analog, time delayed audio signal is described in U.S. Pat. No. 6,178,317. Additionally, U.S. Pat. No. 6,590,944 describes a technique for aligning the digital portion of the radio broadcast signal with the analog portion of the radio broadcast signal using a delay control that provides a DAB signal processing method including diversity delay, interpolation, and blend functions that can be implemented using programmable DSP chips operating in non-real-time. The inventors of the present disclosure have found that it may be desirable to align digital and analog signals in real time, without interpolation, and/or to a higher degree of alignment, e.g., within +/−three audio samples of accuracy.